Centralizers are commonly secured at spaced intervals along a casing or tubing string to provide radial stand-off of the casing or tubing from the interior wall of a borehole in which the string is subsequently installed. The centralizers generally comprise generally aligned collars defining a bore there through for receiving the casing, and a plurality of angularly-spaced ribs that project radially outwardly from the casing string to provide the desired stand-off from the interior wall of the borehole. Centralizers ideally center the casing within the borehole to provide a generally uniform annulus between the casing string exterior and the interior wall of the borehole. This centering of the casing string within the borehole promotes uniform and continuous distribution of cement slurry around the casing string during the subsequent step of cementing the casing string within an interval of the borehole. Uniform cement slurry distribution results in a cement liner that reinforces the casing string, isolates the casing from corrosive formation fluids, and prevents unwanted fluid flow between penetrated geologic formations.
A bow-spring centralizer is a common type of centralizer that employs flexible bow-springs as the ribs. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned collars that are coupled one to the other by a plurality of bow-springs. The flexible bow-springs are predisposed to deploy and bow radially outwardly away from the axis of the centralizer to engage the interior wall of the borehole and to center a casing received axially through the generally aligned bores of the collars. Configured in this manner, the bow-springs provide stand-off from the interior wall of the borehole, and may flex or collapse radially inwardly as the centralizer encounters borehole obstructions or interior wall of the borehole protrusions into the borehole as the casing string is installed into the borehole. Elasticity allows the bow-springs to spring back to substantially their original shape after collapsing to pass a borehole obstruction, and to thereby maintain the desired stand-off between the casing string and the interior wall of the borehole.
Some centralizers include collars that move along the length of the casing in response to flexure of the bow springs. For example, U.S. Pat. No. 6,679,325 discloses, in part, a low-clearance centralizer having an extendable collar at each end, each extendable collar comprising a moving collar and a stop collar that cooperate to form an extendable collar. The extendable collar at each end of the centralizer of the '325 patent includes a longitudinal bore within the aligned extendable collars for receiving the casing to which the stop collars are secured to position the centralizer on the casing. Each moving collar has a collet with a radially outwardly flanged portion for being movably received within an interior circumferential groove or bore within the mating stop collar. A plurality of flexible bow springs are secured at each end to a moving collar, and the two moving collars are maintained in a variable spaced-apart relationship by the bow springs and the stop collars.
A shortcoming of the centralizer of the '325 patent is that the stop collar and the moving collar require axially overlapping structures in order to slidably interface one with the other. This overlapping structure adds to the radial thickness of a centralizer of comparable strength, thereby increasing the minimum collapsed diameter of the casing centralizer and limiting the borehole restrictions through which the centralizer and a casing can pass.
The radial thickness added to the exterior of a casing string by an installed centralizer is but one factor to be considered in selecting a centralizer for a given application. The cost of manufacturing the centralizer is also an important consideration. Many movable collars require the manufacture of complicated mechanisms as compared with simple stationary collars. Even less complicated designs include moving collars that are assembled using multiple components, each of which must be separately manufactured and subsequently assembled into a moving collar. While the end result is useful, the costs of manufacturing multiple components, and the costs associated with assembling the components into a centralizer, make these devices relatively expensive. Thus, there is an ongoing need for centralizers having extendable collars that are radially thinner, but less expensive to manufacture and assemble.